A discovery by astronomers using the Hubble Space
Telescope supports the notion that the Universe is filled with
a mysterious form of energy pushing galaxies apart at an ever-increasing
rate.

April
3, 2001 -- NASA's Hubble
Space Telescope has spotted a burst of light from an exploding
star located much farther from Earth than any previously seen
- a supernova blast in the early Universe that is casting light
on a mystery of truly cosmic scale.

This stellar explosion is extraordinary not only because of
its tremendous distance -- 10 billion light-years from our planet
-- but also because it greatly bolsters the case for the existence
of a mysterious form of "dark energy" pervading the
cosmos. The concept of dark energy, which shoves galaxies away
from each other at an ever-increasing speed, was first proposed,
then discarded, by Albert Einstein early in the last century.

Above: This image from the Hubble
Space Telescope shows the supernova that is providing new evidence
for the theory that a mysterious form of "dark energy"
is causing the expansion of the universe to accelerate. Click
on the image for a picture showing context.

The Hubble discovery also reinforces the startling idea that
the universe only recently began speeding up -- it offers the
first tantalizing observational evidence that gravity began slowing
down the expansion of the universe after the Big Bang, and only
later did the repulsive force of dark energy win out over gravity's
grip.

The team of astronomers, led by Adam Riess of the Space
Telescope Science Institute (STScI), made the discovery by analyzing
hundreds of images taken by Hubble to study how galaxies formed.

"This supernova appears to be one of a special class
of explosions that allows astronomers to understand how the universe's
expansion has changed over time, much as the way a parent follows
a child's growth spurts by marking a doorway," said Riess.
"It shows us the universe is behaving like a driver who
slows down approaching a red stoplight and then hits the accelerator
when the light turns green."

The record-breaking supernova appears relatively bright, a
consequence of the Universe slowing down in the past (when the
supernova exploded) and accelerating only recently.

"Long ago, when the light left this distant supernova,
the universe appears to have been slowing down due to the mutual
tug of all the mass in the universe," explained Riess. "Billions
of years later, when the light left more recent supernovas, the
universe had begun accelerating, stretching the expanse between
galaxies and making objects in them appear dimmer."

Above and Below:
Astronomers
know the universe is expanding because of the "red shift"
seen in the light from distant galaxies. Due to the Doppler Effect,
light waves are compressed when a light source is moving toward
you and stretched when it's moving away. This happens to sound
waves too, which is why the pitch of a police car's siren drops
as the car passes you. With light, however, frequency equates
with color, not pitch. Higher frequencies look bluer and lower
frequencies look redder. So the "red shift" in the
light from the galaxies around ours means those galaxies are
moving away from us. How could all the galaxies be moving
away from us? Only if the universe itself is expanding, as demonstrated
by the balloon below.

"Hubble's ability to find titanic stellar explosions
at these extreme distances is what it takes to confirm this theory
that the universe must have been slowing down before it switched
into high gear," said Dr. Anne Kinney, Director of NASA's
Origins program at NASA Headquarters, Washington, DC. "Later
this year astronauts will install a new camera on Hubble that
will give us 10 times better resolution than the current camera,
which will give us even better capability to find answers to
grand cosmic questions like this."

Observations of several distant supernovae by two teams of
astronomers in 1998 led to the theory that the universe got the
"green light" to accelerate when it was half its present
age. Astronomers say the new Hubble findings rule out other explanations.

Nearly
a century ago, Einstein's Law of General Relativity concluded
the universe must collapse under the relentless pull of gravity.
However, like many scientists of his time, he assumed the universe
to be static and unchanging. To make his equations fit those
assumptions, Einstein added something he called the "cosmological
constant" whose gravity is repulsive, though he had no idea
if it was real.

Left: Albert Einstein revamped humanity's
understanding of the universe in several fundamental ways. Special
Relativity overhauled our notions of space and time, and General
Relativity modified those ideas to include an explanation of
the hitherto mysterious force of gravity. Now it appears his
conjecture that the universe is pervaded by an expansive force
called the "cosmological constant" was also correct.

Shortly afterwards, astronomer Edwin Hubble made the celebrated
discovery that the universe was expanding. He assumed that the
universe must be slowing down under gravity and might even come
to a halt, leading Einstein later to say that his cosmological
constant was the biggest blunder of his career. Now it appears
Einstein was on the right track after all.

The source of the repulsive gravity may be something akin
to Einstein's cosmological constant -- referred to as the energy
of the "quantum vacuum," a subatomic netherworld pervading
space -- or it may be something entirely new and unexpected.

"While we don't know what dark energy is we are certain
that understanding it will provide crucial clues in the quest
to unify the forces and particles in the universe, and that the
route to this understanding involves telescopes, not accelerators,"
said astrophysicist Michael Turner of the University of Chicago.

Riess made the discovery in collaboration with Peter Nugent
(Lawrence Berkeley National Laboratory), Brian Schmidt, (Mount
Stromlo Observatory) and John Tonry (Institute for Astronomy).
NASA's Hubble Space telescope is a project of international cooperation
between NASA and the European Space Agency. The team of astronomers
made their discovery by analyzing hundreds of images taken by
Hubble to study how galaxies formed. Fortuitously, one of those
galaxies contained a supernova previously discovered by astronomers
Ron Gilliland, STScI, and Mark Phillips, Carnegie Institutions
of Washington.

Web Links

The
Hubble Constant -- an explanation of the variable that describes
the rate of expansion -- and thus the age -- of the universe.
From a NASA Space Science Short.

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